People likely to have an affair will do so with or without a website...

Your comment ignores the nature of temptation. These websites have a corrupting influence on those who are not likely to have an affair. They catch the idly curious and change "I wonder what it would be like?" to "That person is available to me." and tempt those who would not be inclined and push them to take action.

Nope, the correct figure of speech is "Free Ryan.":From Wikipedia: "In September 2013, the first book about the Ryan Ferguson case was released: Free Ryan Ferguson: 101 Reasons Why Ryan Ferguson Should Be Released, by Brian D'Ambrosio."

being that many of them won't pass on their genes, nature will take care of it

The pertinent question is "Will nature breed it out before society collapses?" The burden of a generation of men who aren't fulfilling the role that society requires of them is probably too much to bear.

Expecting evolutionary processes to solve societal problems is similar to saying that global warming will be solved by the heat death of the universe.